Carbon Material-Reinforced Polymer Composites for Bipolar Plates in Polymer Electrolyte Membrane Fuel Cells

Bipolar plates (BPs) are one of the most important components of polymer electrolyte membrane fuel cells (PEMFCs) because of their important role in gas and water management, electrical performance, and mechanical stability. Therefore, promising materials for use as BPs should meet several technical targets established by the United States Department of Energy (DOE). Thus far, in the literature, many materials have been reported for possible applications in BPs. Of these, polymer composites reinforced with carbon allotropes are one of the most prominent. Therefore, in this review article, we present the progress and critical analysis on the use of carbon material-reinforced polymer composites as BPs materials in PEMFCs. Based on this review, it is observed that numerous polymer composites reinforced with carbon allotropes have been produced in the literature, and most of the composites synthesized and characterized for their possible application in BPs meet the DOE requirements. However, these composites can still be improved before their use for BPs in PEMFCs.


Introduction
The modern society heavily relies on fossil fuel energy.However, this energy source is finite, and the byproducts of fossil fuels are associated with environmental problems such as climate change [1,2].Therefore, we must move toward renewable energy sources to reduce the impact of anthropogenic activities associated with conventional energy conversion and production.In this direction, hydrogen can be crucial as a clean energy carrier with higher energy density compared with conventional fuels [3,4].Although hydrogen is the most abundant element in the universe [5], it is not the primary energy source available on the earth.Therefore, various technologies have been developed for its production [6,7], storage [8,9], and use [10,11] in an efficient and safe manner.
For hydrogen utilization, polymer electrolyte membrane fuel cells (PEMFCs) are gaining considerable importance because they allow for a highly efficient conversion of the chemical energy contained in hydrogen into electrical energy [11][12][13].However, we must enhance the performance and reduce the cost of several of their components to achieve massive use of PEMFCs [11][12][13].In particular, bipolar plates (BPs) are one of the components that have attracted attention because of their importance in the gas and water management, electrical performance, and mechanical stability of PEMFCs [14][15][16].Therefore, promising materials for use as BPs should meet several technical targets established by the United States Department of Energy (DOE) [17].
Polymers 2024, 16, 671 2 of 24 Thus far, many materials have been investigated and used for the design of BPs.Graphite is the most widely used material for BPs because of its satisfactory corrosion resistance, high thermal and electrical conductivities, and stable chemical properties, among other properties [15,16].However, it suffers several drawbacks such as limited mechanical properties (brittleness), high weight and volume, high manufacturing cost, and poor machinability [15][16][17][18][19]. Therefore, to address these drawbacks, carbon materialreinforced polymer composites have been widely studied as BPs because they offer several advantages such as a light weight, easy machinability, and satisfactory corrosion resistance [15,[18][19][20][21][22].Thus far, polymer matrices have been reinforced via various types of carbon allotropes such as graphite, graphene, multiwalled carbon nanotubes (MWCNTs), and carbon fibers [15,16,[18][19][20][21][22].Several of the proposed composites meet the DOE requirements.In the light of the importance that composites have gained, several review articles have analyzed the use of carbon material-reinforced composites as BPs in PEM-FCs [15,16,[18][19][20][21][22][23][24].In 2017, the generalities of various types of carbon-polymer composites were revised [19].Recently, various models for predicting the electrical conductivity of conductive polymer composites were analyzed [20].In another review, various materials studied as promising candidates for BPs were reviewed, including polymer-based composites [21].More recently, a comprehensive review of the current investigation on various materials used for developing polymer composites for BPs was conducted [22].However, a detailed review focused on the properties of carbon-reinforced polymer composites as BPs materials still does not exist.Therefore, in this review, we present the progress on the use of carbon material-reinforced composites as BPs materials in PEMFCs.

Carbon-Reinforced Polypropylene Composites
Polypropylene has rapidly gained immense popularity in BPs because it is very cheap and flexible for molding and offers satisfactory mechanical properties with relatively decent resistance to impacts compared with other polymers [50][51][52].However, it has a high electric resistance, oxidative degradation, and poor low-temperature impact

Carbon-Reinforced Polyphenylene Sulfide Composites
Polyphenylene sulfide (PPS) has excellent chemical resistance, low degradation at high temperatures, and high rigidity.It also shows remarkable fatigue endurance and creep resistance, which have attracted extensive attention to it regarding its use for BPs [72][73][74].However, PPS has a low elongation to break and low conductivity [75,76].Therefore, to be used for BPs, it must be reinforced with carbon allotropes.In the first instance, PPS was reinforced with a carbon allotrope (see Table 4) [77][78][79][80].For instance, PPS-graphite composites were studied at different concentrations [77].Upon increasing the concentration of graphite in the composites, the in-plane conductivity increased, whereas the flexural strength decreased.A similar trend was observed in the electrical properties of PPS-mesocarbon composites [80].In another study, PPS-graphene composites were produced at different ratios [79].Upon increasing the concentration of graphene in the composites, the in-plane conductivity increased, whereas the flexural strength exhibited an oscillatory behavior [79].
In a bid to further improve the electrical and mechanical properties of PPS-based composites, this polymer has been reinforced with two carbon allotropes with promising results [78][79][80].For example, PPS was reinforced at different ratios of graphite-carbon black.The through-plane conductivity tended to increase upon increasing the concentration of carbon black in the composites, which shows the importance of carbon black in the composites [78].In addition, the PPS polymer was reinforced with different compositions of carbon black-graphene [79].The flexural strength increased as the composition of graphene increased in the composites.The in-plane conductivities obtained for these composites are considerably different from the targets established by the DOE.On the corrosion properties for these composites, the corrosion properties of PPS-graphite-carbon black composites were investigated by varying the composition of the graphite and carbon black [78].The corrosion properties obtained for the PPS-graphite-carbon black composites were similar to those required by the DOE [78].Various PPS-based composites reinforced with one or two carbon allotropes meet the electrical and mechanical properties required by the DOE.However, the studies developed to date are still scarce.

Carbon-Reinforced Polybenzoxazine Composites
The polybenzoxazine polymer has good thermal properties.However, this material exhibits high brittleness, which makes it difficult to use them to prepare films or complex structures [81][82][83].Carbon allotrope-reinforced polybenzoxazine composites were proposed to improve the mechanical properties and processibility of these composites [84][85][86].Thus far, some studies have been conducted on the use of polybenzoxazine composites reinforced with different types of carbon allotropes as materials for BPs (see Table 5) [87][88][89][90].For instance, polybenzoxazine-graphite composites at different compositions of graphite were studied.With the increasing concentration of graphite in the composites, the in-plane conductivity increased, whereas the flexural strength decreased [87,88].A similar trend was observed for polybenzoxazine-graphene composites [90].Interestingly, the mechanical and electrical properties obtained for most of these composites were higher than those required by the DOE [87][88][89][90].
To reduce the graphite content in polybenzoxazine-graphene composites, as for other polymers, the strategy of incorporating other carbon allotropes in the composites has been established.Some studies were conducted on polybenzoxazine-graphene composites reinforced with two or three carbon allotropes.For instance, polybenzoxazine-graphite composites were reinforced with different concentrations of graphene [89].The in-plane conductivity increased upon increasing the concentration of graphene in the composites.However, a higher flexural strength was observed in the polybenzoxazine 17%-graphite 80.5%-graphene 2.5% composite [89].In another study, polybenzoxazine-graphite-graphene composites were reinforced with different concentrations of MWCNTs [91].The in-plane conductivity increased as the concentration of MWCNTs increased in the composites.In addition, the electrical properties obtained for polybenzoxazine-graphite-graphene-MWCNTs were higher than those for polybenzoxazine-graphite-graphene ones [91].These studies demonstrate the importance of incorporating more carbon allotropes into polybenzoxazinegraphite composites [89,91].Various in polybenzoxazine-based composites reinforced with one or two carbon allotropes meet the electrical and mechanical properties required by the DOE.However, the investigations developed to date are still scarce.

Carbon-Reinforced Epoxy Resin Composites
Epoxy resin is also considered a polymer matrix for BPs because of its remarkably high adhesion strength, satisfactory heat resistance, good chemical and mechanical stabilities, easy mass production, and cost effectiveness [92].However, epoxy resin BPs must exhibit better mechanical, electrical, and corrosion resistance properties to be applied as BPs [93].Therefore, epoxy is generally reinforced with carbon allotropes to enhance these properties [24].Recently, various studies have been conducted on epoxy resin reinforced with carbon allotropes [56,92,[94][95][96][97][98], highlighting the use of graphite.Various ratios of epoxy resin-graphite were employed, ranging from ~20% to 80% graphite.Interestingly, the effect of the composition of epoxy resin-graphite composites on their electrical and mechanical properties was explored in detail.For instance, epoxy resin-graphite composites with different ratios (60:40, 50:50, and 40:60 wt.%) were investigated [94].The in-plane conductivity, and flexural strength tended to increase upon increasing the concentration of graphite in the composites.A similar trend was observed in another study, in which different ratios of epoxy resin and graphite (40:60, 30:70, and 20:80 wt.%) were explored [96].Interestingly, various synthesized materials comply with the DOE requirements [92,94,97].

Synthesis Methods
So far, several carbon allotrope-reinforced polymer composites have been produced (see Tables 1-8), where several of these synthesized composites have the same or similar compositions.However, their mechanical and electrical properties differ substantially.These differences can be attributed to the synthesis conditions employed to produce these composites.Almost all analyzed polymer composites were produced using the compression molding technique (Figure 2).This method uses some parameters that have an influence on the characteristics and properties of the synthesized composites such as molding time, molding temperature, and molding pressure [104].Therefore, it is important to consider these parameters for the production of polymer composites reinforced with carbon allotropes.(20,30,40,60,90, and 120 min) [87].The maximum conductivity (228 S/cm) was measured at 60 min of molding time.While the maximum flexural strength (48 MPa) was obtained at 90 min [87].
Molding temperature: It has been documented that molding temperature substantially changes the electrical and mechanical properties of polymer composites reinforced with carbon allotropes [30,87,96].For instance, the conductivity of a phenolic resin (15 wt.%) composite reinforced with graphite (85 wt.%) changed from 108 S/cm to 142 S/cm when the molding temperature increased from 220 to 240 °C, whereas the flexural strength increased from 53 MPa to 62 MPa when the temperature presented the same increase [30].In another study, the conductivity of polybenzoxazine (15 wt.%) composite reinforced with graphite (85 wt.%) increased from 234 to 247 S/cm when the temperature changed from 160 °C to 200 °C.Also, the flexural strength increased from 34 to 44 MPa when the temperature increased from 160 °C to 200 °C [87].
Molding pressure: Some studies have demonstrated that the electrical and mechanical properties are directly related to the molding pressure [31,39,42,96,100].For example, the conductivity and flexural strength of phenolic resin composites increased when the molding pressure increased [31,39,42].The same trends were observed for epoxy resin composites [96,100].Molding time: It has been reported that this parameter substantially changes the characteristics and properties of polymer composites reinforced with carbon allotropes [30,87].For instance, the electrical and mechanical properties of phenolic resin (15 wt.%) composites reinforced with graphite (85 wt.%) were investigated by varying the molding time (15,30,45,60,75, and 90 min) [30], and the best results were found with 60 min of molding time (142 S/cm and 61.6 MPa).In another study, polybenzoxazine (15 wt.%) composites reinforced with graphite (85 wt.%) were produced by varying the molding time (20,30,40,60,90, and 120 min) [87].The maximum conductivity (228 S/cm) was measured at 60 min of molding time.While the maximum flexural strength (48 MPa) was obtained at 90 min [87].
Molding temperature: It has been documented that molding temperature substantially changes the electrical and mechanical properties of polymer composites reinforced with carbon allotropes [30,87,96].For instance, the conductivity of a phenolic resin (15 wt.%) composite reinforced with graphite (85 wt.%) changed from 108 S/cm to 142 S/cm when the molding temperature increased from 220 to 240 • C, whereas the flexural strength increased from 53 MPa to 62 MPa when the temperature presented the same increase [30].In another study, the conductivity of polybenzoxazine (15 wt.%) composite reinforced with graphite (85 wt.%) increased from 234 to 247 S/cm when the temperature changed from 160 • C to 200 • C. Also, the flexural strength increased from 34 to 44 MPa when the temperature increased from 160 • C to 200 • C [87].
Molding pressure: Some studies have demonstrated that the electrical and mechanical properties are directly related to the molding pressure [31,39,42,96,100].For example, the conductivity and flexural strength of phenolic resin composites increased when the molding pressure increased [31,39,42].The same trends were observed for epoxy resin composites [96,100].

Production Costs
Polymer composites reinforced with carbon allotropes are excellent candidates for use in BPs because their properties are superior to those required by the DOE.However, some of the carbon allotropes (e.g., MWCNTs and graphene) utilized to reinforce polymer matrices present challenges related to production costs.It is well documented in the literature that the production methods used to produce these carbon allotropes are still expensive because these structures were discovered recently [105][106][107].For the year 2025, the DOE established cost targets of 2 USD/kW for BPs in PEMFCs [17,108].Considering the current costs of graphene and MWCNTs, their real applications in BPs could be limited since BPs based on graphene-reinforced polymer materials are more expensive than graphite and metal BPs, and their costs could be much higher than those established by the DOE.Therefore, to ensure the use of composite materials reinforced with graphene and MWCNTs, it is necessary to have a method that allows for the production of these carbon structures in large quantities and with good quality, which could help to use these materials in BPs and, thus, comply with DOE's cost targets.

Stability of BPs
The thermal stability of polymer composites reinforced with carbon allotropes is important for their use in BPs.However, it is well known that polymer-based composites can exhibit degradation problems at high temperatures.Therefore, it is essential to know the thermal stability of polymer composites reinforced with carbon allotropes at the PEMFCs operating temperatures (80-120 • C).Fortunately, there have been studies on the thermal stability of polymer composites reinforced with carbon allotropes at PEMFCs operating temperatures, and the results are promising [42,44,49,88].For instance, the phenolic resin (45 wt.%) and graphite (55 wt.%) composite presented a 2.2 wt.% loss at 400 • C [42].Also, the storage modulus was practically constant in a range from 30 to 100 • C [42].In another study, the thermal stability of phenolic resin (varying the concentration) composites reinforced with exfoliated graphite (varying the concentration), carbon black (5 wt.%), and graphite (3 wt.%) was studied at 200 • C. The best results (0.03 wt.% loss) were obtained with the phenolic resin (57 wt.%)-exfoliated graphite (35 wt.%)-carbon black (5 wt.%)-graphite (3 wt.%) composite [49].It has also observed that the storage modulus of the synthesized composites were similar when the temperature varied from 30 to 75 • C. Interestingly, it has also been shown that the incorporation of carbon allotropes improves the thermal stability of polymer composites [109][110][111][112][113][114][115].According to the studies conducted on the thermal stability of polymer composites reinforced with carbon allotropes, these may not present serious degradation problems and may practically maintain the mechanical properties (storage modulus) at the operating temperatures of PEMFCs.

Conclusions and Future Directions
Carbon material-reinforced-polymer composites have been widely studied as BPs because they offer several advantages, such as a light weight, easy machinability, and a satisfactory corrosion resistance.From this detailed review, the following conclusions and future directions can be suggested: (a) For single-polymer composites reinforced with carbon allotropes, phenolic resin, polypropylene, PPS, polybenzoxazine, and epoxy resin are the polymers more commonly used for BPs.However, more studies are required for PPS, polybenzoxazine, and epoxy resin-based composites since the studies developed to date show promising results.(b) The single-polymer composites have been reinforced using various types of carbon allotropes, mainly graphite, carbon fibers, carbon black, carbon nanotubes, and graphene.However, it is necessary to extend the study on single-polymer composites reinforced with carbon nanotubes and graphene since these are popular in the literature for their extraordinary electrical and mechanical properties.
(c) Two-polymer composites with one, two, or three carbon allotropes have been partially explored with outstanding results.Therefore, more detailed studies on these composites should be conducted.(d) Almost all composites were produced using the compression molding technique.
Nevertheless, the use of additive manufacturing could be a good strategy to produce BPs using the composites analyzed in this review.(e) Future studies should report on the properties required by the DOE and, thus, facilitate the analysis of the results.

Figure 1 .
Figure 1.Schematic of electrical conduction mechanism in polymer composite containing (a) single and (b) multiple carbon fillers.Reproduced with permission from Reference [49].

Figure 1 .
Figure 1.Schematic of electrical conduction mechanism in polymer composite containing (a) single and (b) multiple carbon fillers.Reproduced with permission from Reference [49].

Table 1 .
Electrical and mechanical properties of phenolic resin reinforced with an allotrope of carbon.

Table 2 .
Electrical and mechanical properties of phenolic resin reinforced with two and three carbon allotropes.

Table 3 .
Electrical and mechanical properties of polypropylene reinforced with two and three allotropes of carbon.

Table 4 .
Electrical and mechanical properties of polyphenylene sulfide reinforced with an allotrope of carbon.

Table 5 .
Electrical and mechanical properties of polybenzoxazine reinforced with an allotrope of carbon.

Table 6 .
Electrical and mechanical properties of epoxy resin reinforced with two allotropes of carbon.

Table 7 .
Electrical and mechanical properties of two polymers reinforced with an allotrope of carbon.

Table 8 .
Electrical and mechanical properties of two polymers reinforced with two and three allotropes of carbon.